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Not all types of asbestos are strongly implicated, with crocidolite being the main causative fibre type. Not surprisingly, given the sources of asbestos exposure being predominantly mining, construction, lagging and machinery mechanics, 60-80% of cases are encountered in males, in general 20 to 35 years after exposure 1,5-6. Some areas of the world have very regional hotspots, such as Belfast in Northern Ireland, due to the historic ship building industry.

There has been no convincing evidence for an association with smoking 6.

Clinical presentation

Typically patients present with dyspnoea and low posterior non-pleuritic chest pain. Pleural effusions are seen in the vast majority of patients at some stage during their disease. Up to 25% of patients have metastatic disease at the time of presentation if staged with FDG PET 5.

Pathology

There are three histological types of mesothelioma:

epithelial: ~60% *

mixed: 25%

sarcomatoid: 15%

The cytological and histological diagnosis can be difficult, with mesothelial hyperplasia and metastatic adenocarcinoma appearing similar. Specific markers are helpful including:

Radiographic features

Plain film

Chest radiographs are of limited utility and are non-specific 6, demonstrating a pleural opacity which may extend around and encase the lung. Reduction in volume of the affected hemithorax is common resulting in shift of the mediastinum towards the lesion 4.

Rib destruction or extension beyond the lateral and anterior margins of the chest wall may be evident. Mediastinal lymph node enlargement and pleural effusion may also be seen.

CT

CT is the most commonly used modality for the assessment of mesothelioma and is able to stage the disease accurately in most patients.

The appearance is that of a soft tissue attenuation nodular mass which spreads along pleural surfaces including into pleural fissures and often creating a pleural rind.

Calcification is seen in 20% of cases which usually represents engulfed calcified pleural plaques rather than true tumour calcification 4. Sarcomatoid variants may demonstrate osteosarcoma or chondrosarcomatous components which may also be calcified.

Mesotheliomas have a predilection for direct invasion of adjacent structures (chest wall, diaphragm and mediastinal content) but also frequently metastasise to the contralateral lung and local nodes1-2,4.

To confidently predict chest wall invasion the extrapleural fat plane should be seen to be infiltrated and/or direct extension in bone or muscle identified 4.

MRI

MRI, although not routinely used, may have a role in refining the staging and better delineating the extent of the disease in surgical candidates especially with regard to chest wall and diaphragmatic invasion 4.

PET

In addition there appears to be a correlation between the degree of FDG uptake and the biological aggressiveness of the tumour, which may help to guide treatment 4.

Treatment and prognosis

Treatment continues to be challenging and long term survival is poor. Single modality treatment (surgery, radiotherapy, chemotherapy, immunotherapy and even photodynamic therapy) have not be shown to improve survival 3. More recently multi-modality treatment has had some impact on favourable sub groups (early disease, and epithelioid histology). Treatment includes:

extrapleural pneumonectomy

adjuvant chemotherapy

radiotherapy

The prognosis is poor for all tumour types with an overall median survival without treatment of 4-12 months 3. In favourable patient sub-groups up to 45% 5 year survival may be achievable 3, however even with aggressive multi-modality therapy overall 5 year survival remains poor (3-18%) 3 with a median survival time of approximately 18 months 4.

Differential diagnosis

The differential is dependant on the exact nature of tumour involvement and the modality. General imaging differential considerations include